CWE-323: Reusing a Nonce, Key Pair in Encryption

What is Reusing a Nonce, Key Pair in Encryption?

• Overview: Reusing a nonce or key pair in encryption is a vulnerability where the same nonce or key is used more than once, which can compromise the security of the encryption process. A nonce (number used once) is intended to be unique for each encryption operation to ensure that identical plaintext messages produce different ciphertexts.

• Exploitation Methods:

• Attackers can exploit this vulnerability by analyzing patterns in ciphertexts generated using the same nonce, which can lead to the recovery of the plaintext or encryption keys.
• Common attack patterns include replay attacks, where the attacker uses repeated nonces to deduce encryption keys, and known-plaintext attacks, which leverage repeated nonces to decrypt messages.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access to sensitive data and potential decryption of confidential information.
• Potential cascading effects involve further compromise of data integrity and confidentiality across systems relying on the affected encryption.
• Business impact can be severe, potentially resulting in data breaches, loss of customer trust, regulatory fines, and reputational damage.

• Prevention Guidelines:

• Specific code-level fixes include implementing logic to generate a unique nonce for every encryption operation, ensuring no reuse within the lifetime of the encryption key.
• Security best practices involve using well-established cryptographic libraries that handle nonce generation automatically and securely.
• Recommended tools and frameworks include utilizing cryptographic libraries like OpenSSL or libsodium, which provide secure nonce management, and implementing checks to detect and prevent nonce reuse.

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Technical Details

Likelihood of Exploit: High

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Nonces are often bundled with a key in a communication exchange to produce a new session key for each exchange.

Vulnerable Code Example

from Crypto.Cipher import AES
import os

class EncryptionService:
    def __init__(self, key):
        self.key = key
        self.nonce = b"fixednonce"  # Vulnerable: Using a fixed nonce

    def encrypt(self, plaintext):
        cipher = AES.new(self.key, AES.MODE_GCM, nonce=self.nonce)  # Reuse of nonce
        return cipher.encrypt(plaintext)  # Reuse of cipher leads to nonce reuse

    def decrypt(self, ciphertext):
        cipher = AES.new(self.key, AES.MODE_GCM, nonce=self.nonce)  # Reuse of nonce
        return cipher.decrypt(ciphertext)  # Reuse of cipher leads to nonce reuse

Explanation of Vulnerability:

In this vulnerable code, a fixed nonce (b"fixednonce") is used for all encryption operations. This is a critical security flaw because using the same nonce with the same key for multiple encryptions can lead to severe vulnerabilities. It can allow attackers to perform attacks such as:

• Plaintext Recovery: If two messages are encrypted with the same key and nonce, an attacker can deduce information about the plaintexts.
• Replay Attacks: Reusing the same nonce can allow attackers to replay previously captured ciphertexts.

How to fix Reusing a Nonce, Key Pair in Encryption?

To fix the issue of reusing a nonce, follow these steps:

1. Use a Unique Nonce for Each Encryption Operation: Generate a random nonce for every encryption operation to ensure that the same key and nonce pair is not reused.
2. Securely Store Nonce with Ciphertext: Store the nonce alongside the ciphertext, as it is needed for decryption.
3. Use Secure Random Number Generators: Use cryptographically secure random number generators to create nonces.

Fixed Code Example

from Crypto.Cipher import AES
import os

class EncryptionService:
    def __init__(self, key):
        self.key = key

    def encrypt(self, plaintext):
        nonce = os.urandom(12)  # Generate a new random nonce for each encryption
        cipher = AES.new(self.key, AES.MODE_GCM, nonce=nonce)  # Use the new nonce
        ciphertext, tag = cipher.encrypt_and_digest(plaintext)  # Ensure integrity with a tag
        return nonce + tag + ciphertext  # Concatenate nonce and tag with ciphertext for storage

    def decrypt(self, nonce_and_tag_and_ciphertext):
        nonce = nonce_and_tag_and_ciphertext[:12]  # Extract nonce from the beginning
        tag = nonce_and_tag_and_ciphertext[12:28]  # Extract tag
        ciphertext = nonce_and_tag_and_ciphertext[28:]
        cipher = AES.new(self.key, AES.MODE_GCM, nonce=nonce)  # Use extracted nonce for decryption
        return cipher.decrypt_and_verify(ciphertext, tag)  # Decrypt and verify the integrity

Explanation of Fix:

In the fixed code:

• A new random nonce is generated for each encryption operation using os.urandom(12), ensuring that the nonce is unique for every operation.
• The nonce is securely concatenated with the ciphertext and an authentication tag, which is necessary for verifying the integrity of the ciphertext during decryption.
• During decryption, the nonce and tag are extracted from the stored data, ensuring that decryption uses the correct parameters.
• This approach prevents the reuse of nonce-key pairs, securing the cryptographic operations against the vulnerabilities mentioned above.